1. Field of the Invention
The present invention relates to a method for diagnosing breakdowns in a production line.
2. Description of the Prior Art
A so-called sequential control has been generally well known and is a method for sequentially controlling an operating system of equipment in accordance with operating programs for every output means (for example, an actuator) constituting the operating system. Meanwhile, in order to allow this sequential control to fully function, various arrangements have been made to supervise the controlling state to thereby diagnose an occurrence of an accident or breakdown of the equipment. For one example of such an arrangement, Japanese Patent Application Laid-open Publication 60-238906 discloses an abnormal operation detecting device of a so-called teaching playback method. In this prior art abnormal operation detecting device, an operating pattern for each component in a sequential controlling circuit of the equipment when the equipment is normally operated is stored in advance, so that the stored operating pattern is sequentially compared with a pattern obtain during actual operation of the equipment. Therefore, when the patterns, i.e., the stored normal pattern and actually-obtained pattern are not coincident, the detecting device determines that something is abnormal in the equipment.
The diagnosing method of accidents in the teaching playback manner employed in the above-described device has, however, such a drawback that an operating pattern, if it is not coincident with the pattern stored in the device, is diagnosed to be abnormal even when the equipment is actually normally operating. In other words, although the pattern indicative of the normal operation of the equipment is possibly diversified, a pattern of the equipment to be stored in the device is restricted (generally, only one kind of pattern is stored), and accordingly, it cannot be avoided that any pattern different from the stored pattern is diagnosed abnormal. As a result, the detection or diagnosis may often result in error.
Moreover, according to the prior art diagnosing method, generally, one sequence circuit is formed for one actuator so as to detect abnormalities. The diagnosis of breakdowns or abnormalities is done for every individual actuator, based on which the equipment is diagnosed as a whole. Because of this structure, an abnormality detecting circuit for the control unit should have equal or greater capacity than the control unit, resulting in a bulky and complicated construction. Moreover, it is laborious that when some alterations are made on the equipment, then, the diagnosing program should be changed simultaneously. Even when the actuator is not so greatly damaged and accordingly does not affect the entire operation of the equipment, e.g., a secular change causes some delay in the operation of the actuator, the equipment itself is erroneously diagnosed to be abnormal. Furthermore, when the cycle time of the equipment is disturbed because of an inadvertent manipulation by an operator, but without bringing the actuator into disorder, the fact cannot be detected by the prior art diagnosing method.